Amr Abdelhamid demonstrated at least as early as 1981 that throttling the diffuser exit can change flow angle in a vaneless diffuser. See, Advanced Concepts in Turbomachinery, August 1981, Fluid Dynamics Institute, Hanover, N.H. It has also been shown that throttling the diffuser inlet could have performance benefits. Inlet throttling in a vaned diffuser is shown in U.S. Pat. No. 4,378,194 issued Mar. 29, 1983 to Bandukwalla. Throttling the entire diffuser passage through a slotted wall is shown, for example, in U.S. Pat. No. 4,403,914 issued Sept. 13, 1983 to Rogo et al. However these prior art devices are characterized by certain shortcomings. Inlet throttling where the flow has a high mach number causes sudden expansion loss, turbulence and high friction losses. Slotted wall designs are costly, complex and subject to leakage losses. The throttled exit vaneless design is mechanically simple but not efficient.